Precarburization and other prediffusion treatment of spiral rolled or differentially plastically formed and standard type drill steel



United States Patent PRECARBURIZATION AND OTHER PREDIFFU- SION TREATMENTOF SPIRAL ROLLED 0R DIF- FERENTIALLY PLASTICALLY FURMED AND STANDARDTYPE DRILL STEEL Tadeusz W. Wlodek, 297 5th Ave., Ottawa, Ontario,Canada No Drawing. Filed Oct. 24, 1965, Ser. No. 505,018 8 Claims. (Cl.14812.1)

This application is a continuation-in-part of application Serial No.148,854, filed October 31, 1961, now

abandoned, which is a continuation-in-part of application Serial No.835,409, filed August 24, 1959, now abandoned, which is acontinuation-in-part of application Serial No. 551,455, now Patent No.3,017,697, filed December 6, 1955, and which, in turn, is acontinuation-in-part of Serial No. 373,369, filed August 10, 1953, nowabandoned.

This invention relates to a novel process for the manufacture of drillsteel, hollow drill steel and other steel and metal structural members.More particularly, the invention concerns a method of improving thefatigue and corrosion fatigue resistance of drill steel by a carburizingtreatment, followed by subjecting the steel to differential plasticdeformation at ordinary or at high temperatures, with or withoutsubsequent over-rolling.

This invention introduces also an additional process of precarburizationof drill steel and other steel and metal structural members; and/ orprecoating by thermal diffusion, preimpregnation and preplating andother coating treatments with other than carbon elements, all thesetreatments are called as preimpregnation treatments and all appliedbefore final rolling and forming operations.

In general, it is the practice to form drill steel, and particularlyhollow drill steel, from billets into rods having a cross section whichmay be round, hexagonal, or octagonal, the latter being commonly in theform known as quarter-octagon, by a rolling procedure; The resultingrods may be formed solid or with an internal longitudinal hole. Suchdrill steel rods and similar structural metal members are subject to avariety of stresses during use, which tends to hasten fatigue andfailure of the steel.

In my Patent No. 3,017,697, there is disclosed a novel method forimproving the fatigue resistance of drill rods and similar structuralmembers which comprises subjecting the surface of the rods or metalmembers by cold working to pressures beyond the elastic limit of thesurface portions. The pressures are applied in such a way as to resultin differential plastic deformation of the surface at spaced intervalssufiicient to provide plastically deformed depressions of smalldimensions, for example between one-thousandth of an inch andone-sixteenth of an inch in depth. The intervals are interspaced withsubstantially undeformed portions to provide locked-in residual stresseswhich resist fatigue.

Preferably, the distance between the depressions is kept between aboutone-eighth of an inch and about onesixty-fourth of an inch, the distancebetween the depressions being such that it is greater than the depth ofthe depressions. In this manner, plastic deformation is impressed on themetal surface at spaced intervals thereon, the intervals being so spacedas to leave between adjacent pairs of depressions a portion of the metalwhich has not been subjected to deformation, the interval portions'remaining in the original elastic form. In general, juxtaposition ofregions plastically deformed with regions of lesser plastic deformation,both regions being arranged in a regular or irregular pattern, is thetopic of this treatment. The transition between these two regions may begradual or abrupt, and the built-in residual stresses are icesubstantially perpendicular to the direction of plastically deformedregions, i.e., direction of pattern.

It should be also emphasized that impression of grooves and depressionsprovide, through so-called mutual interlocking phenomena, a controllablemechanism of plastic deformation of treated surfaces, resulting ingreatly improved physical qualities of material treated by thisinvention. These depressions may take the shape of helixes or ofparallel grooves, for example. The grooves or depressions may beimpressed by rolling, spiral rolling, ex truding, pressing, stamping,hammering, squeezing, drawing or twisting, or the like, depending uponthe character of the rod or metal structural member. In the case ofhollow or tubular members, such as pressure vessels or cylinders, thegrooves or depressions may be applied to the inner or outer surfaces orto both.

The grooves or depressions made in the surface of the metal beingplastically deformed should be at least 0.001 inch (0.025 mm.) in depth.Where extremely hard metal is employed, the depressions formed bypressure applied at spaced intervals are very shallow. The preferredrange of depth is from about inch (0.12 mm.) to about inch (0.4 mm). Itwill be apparent that the spacing of the grooves or depressions and thedepth used will be dependent upon the form of groove employed. Thepreferred distance between the grooves is from about A; inch (3.2 mm.)to about & inch (0.4 mm.), the distance generally preferred being about1 inch (1.08 mm.).

While this aspect of the invention is applicable to such structuralmembers as steel, aluminum and magnesium alloy plates, and the like, itis especially applicable to drill steel rods.

In accordance with the first aspect of the present invention, it hasbeen found that drill steel and other structural metal members, whichare subjected tothe differential plastic deformation treatment disclosedin the abovementioned copending applications, are further improved withrespect to their fatigue resistance and other physical properties ifthey are first subjected to carburization, or to carburization togetherwith diffusion treatment, or also to impregnation treatments prior toplastic deformation in the manner described.

It is the present industrial practice to carburize drill steel and otherstructural members only after all rolling and forming operations arecompleted. Thus, a typical process starts with a length of drill steel10 to 20 feet long and from 1 to 1.5 inches in diameter. This is placedin a long canburization furnace packed with carburization mixture, andheated for 8 to 12. hours at about 1700"- 1800 F., then quenched andcleaned. This results in a case hardened steel which can be subjected toplastic deformation by impressing shallow grooves or depressions on itssurface which serve to increase the life of the steel.

In accordance with the second aspect of the invention, it has been foundthat by applying plastic deformation in the form of grooves ordepressions to the surface of a drill steel which has been precarburizedand/or preimpregnate-d with other metals before it is rolled into adrill rod, the same effect or even a heightened effect in improvement offatigue resistance and life of the steel can be obtained with shallowergrooves. This is believed due to the fact that precarburization prior torolling enables the rolling operation to bring about a better and moreeven distribution of the carbon and other alloying elements along thesurface of the drill steel, a greater depth of penetration of the casehardening carbon, and a gen eral diffusion of the carbon within thesteel coupled with refinement of its grain structure, resulting-inincreased strength.

Accordingly, the manufacture of drill steel, hollow drill steel, andother solid and hollow structural members, in accordance with theinvention, is based on precarburiza- 3 tion and/or preimpregnation withother metals and other prediifusion treatments of the exterior, or ofboth the interior and exterior, of such members, before the applicationof final rolling and forming operations. If desired, further diffusionof the precarburization carbon and prediffused alloys can be carried outprior to the final rolling or other operations, in accordance with ourprocedures, or they may also be applied after rolling and forming.

In order to simplify the descriptions, the precarburization,preimpregn-ation, and preditfusion treatments in this applicationidentify treatments applied, as recommended in this invention, i.e.,before final forming and rolling operations. The same treatments appliedafter forming operations are identified as carburization, impregnationand diffusion treatment-s with omission of pre.

Preimp-regnation processes, in general, involve the introduction ofother than carbon hardening elements or alloying substances into aferrous or nonferrous alloys by heating the metal in contact withhardening materials which can be solid, liquid or gas, to a temperatureabove the characteristic transformation range of treated metal, for afew hours, usually from 4 to 8 hours, for a lower temperature rangeabout 100 F. above the characteristic transformation temperature, andfrom 30 min. to 4 hours for a very high temperature range about 100 F.to 500 F. above the characteristic transformation temperature, dependingon the type of metal treated and applied and required thickness of thehardened case, usually from 30 to inch deep. Preimp-regnationtreatments, with other than carbon elements i.e., with such metals asCr, Mo, Si, Ni, Va, Co, U, Ti, columbium, boron and tungsten and theiralloys, recommended for the application in the proposed method ofproduction, e.-g., before final rolling and forming of hollow and soliddrill steel and other structural metal members, include transfertechniques of selected elements into the surface of the prefabricatedbillet. These selected techniques are: chromizing, siliconizing,tungstenizing, in general coating with metals specified above.

Cladding with layer of a selected metal or alloy specified in the abovelist, is accomplished by heating in a granular packing of the metal tobe applied in a protective atmosphere, or by heating in a gas from whichthe metal is deposited, e.-g., from a chloride of the metal. Moreover,cladding for the proposed preimpregnation treatment, with specifiedabove metals, is accomplished by thick electroplating technique, bymetallizing with blowpipe, hot-dipping and electrochemical depositionsfollowed if required by a prediffusion treatment. A typical example of apreimpregnation treatment is given below:

Example I p The preimpregnation with chromium, molybdenum or the like,in accordance with the invention is applied both to solid or hollowdrill steel billets or other structural metal members by thermaldiffusion of selected metals into the surface of treated billets inorder to convert a Zone at the surface of steel into a corrosionandfatigue-resisting alloy. This transformation is effected by theimpregnation of the surface with chromium, molybdenum or other metalslisted above by chemical and physical reactions taking place at a hightemperature. In one particular example of preim-pregnation with chromiumor molybdenum, a treated billet is heated to between 1900 F. and 2000 F.in chromium or molybdenum ferroalloy dust and the surface of billet isenriched with the respective metal. The depth of impregnation depends ontime and temperature of treatment within the range of 4 to 12 hours at2000 F., the required depth of the alloyed case inch being reached.

In another example of chromizing, a steel billet of SAE 1080 drill steelis heated to 1800-2000 F. in a controlled atmosphere furnace in agranular mixture of ferrochromium. Maintaining a reducing atmosphereinside 'the furnace, chromium is brought to the steel surface in theform of a vaporized compound-metal chloride. Normal-1y the time requiredfor this treatment is between 6 to 12 hours in order to obtain thedesired -alloyed case Of %000 iIICh.

The precarburization step will be illustrated with reference to drillsteels with low (0.15-0.2S%) or medium (0.30-0.45 carbon content, whichalso contain one or more other alloying elements such as Ni, Cr, Mo, Mn,Si, U, V, Co, Cb, Cu, W, and B, and also with reference to a No. 1080SAE type plain high carbon steel.

A typical low carbon nickel-chromium-molybdenum drill steel may have,for example, either of the following two compositions:

(1) Carbon 0.15O.25%; chromium 0.400.80% nickel 1.5-2.50%; molybdenum0.20-0.50%; silicon 0.25- 0.35%; manganese 0.400.60%; phosphorus andsulphur 0.02% each, maximum.

. (2) Carbon 0.20-0.30%; chromium ZOO-3.5%; nickel 0.20-0.40%;molybdenum 0.300.60%; silicon 0.30 0.60%; manganese 0.60l.00%;phosphorus and sulphur 0.02% each, maximum.

A typical medium carbon nickel-chromium-molybdenum drill steel suitablefor use in connection with the invention is the following:

Carbon 0.300.45%; chromium 0.300.50%; nickel 2.53.5%; molybdenum0.200-.30%; silicon (MS-0.35%; manganese 0.60l.00% phosphorus andsulphur 0.020% maximum each.

Two typical high carbon plain steels which are adapted to the process ofthe invention are:

(1) Plain carbon steel 0.60-0.80% carbon and small additions of otheralloying elements like Mo, Va, Ca.

(2) Carbon-chromium steel 0.70 to 1.00% carbon and about 1.00% chromiumwith small additions of other elements.

Precarburization rand/or preimpregnation, in accordance with the thirdaspect of the invention, may be applied both to solid drill steel or tohollow drill steel billets. The precarburization may be effected bytreatment with any conventional suitable carburizing medium, Whethergaseous, liquid or solid. Preferably a solid carburizing medium rich incarbon is used. Typical carburizing media are mixtures of hardwoodcharcoal and barium carbonate, such as 2 parts barium carbonate to 3parts wood charcoal, or a molten cyanide salt bath containing 50% sodiumcyanide and 50% sodium carbonate. A typical gaseous medium is carbonmonoxide or a gas rich therein.

The rod, billet or member to be precarburized is packed in a carburizingoven with .a suitable amount of solid medium, depending upon the size ofthe member.

Where a gaseous .medium is used the billet can be precarburized in a gascarburizing furnace employing carbon monoxide, or a hydrocarbon gas suchas methane, ethane, propane or butane, or mixtures of city gas ornatural gas.

The precarburizing temperature employed will range from about 1600 toabout 2300 F., depending upon the medium employed and the type of metalbeing treated. After the required case has been formed, the member maybe heat treated in any desired manner, for example utilizing oil orwater quenching. The precarburization time may be any desired period,preferably from about A to about 8 hours. The thickness of the case maybe any desired thickness, but ordinarily will vary from about 0.02 to0.2 inch.

The fourth aspect of the invention is the application of very hightemperature precarburization, preimpregnation and prediffusiontreatments. The carburization, impregnation and diffusion treatments,all high temperature treatments of previously finally formed structuralelements, are treatments of which the upper limit of temperature islimited in order to avoid undesirable excessive grain growth,dimensional changes, intercrystalline se' regations and other changeswhich effect, very substantiah 5 l'y reduce the fatigue strength anddurability of hollow and solid drill steel, e.g., the upper limit ofcarburization is limited to about 1750 F., and the temperature of otherimpregnation and diffusion treatment specified above is kept as low aspossible above the required characteristic transformation temperaturesof treated metals.

In the new and improved method of the present invention for thefabrication of precarburized and/or preimpre-gn-ated, and prediffuseddrill steel, the upper limit of preca-rburization can be increased from1750 F. up to 2300 F., because final extensive rolling and other plasticdeformations will eliminate the detrimental effects introduced by veryhigh temperature of precarburization, preimpregnation and prediffusiontreatments with other than carbon elements.

Employing intensive and substantial plastic deformation will refine thegrain size and the metallographic structure as a whole. Coarse crystalstructure and concentration of grain boundary intercrystallinesegregations of carbon and other alloying constituents built-up anddeveloped during high temperature treatments will be 'worked through andbetter redistributed during final rolling deformation, an important stepof the invention provided for that purpose. Applied afterprecarburization and/ or preimpregnation, and prediflusion treatments,very effective refining treatment of the coarse structure and largecolumnar grains, through extensive plastic deformation will cause also abetter longitudinal alignment of microand macrofibrous structure intothe direction of final rolling What will: also increase the fatiguestrength of longitudinal fibrous of surface layers.

Such very high temperature treatments from 1800 F. up to 2300 F. forfast precarburization treatments, require only a fraction of time forcompletion of this treatment as compared to standard treatments, whichis a substantial and very important saving in production time andequipment, and a significant improvement in the quality of finalproduct.

Similarly by very high temperature fast preimpregnation andprediffusiontreatments from 100 to 500 F. above the requiredtransformation temperature recommended, the time of treatment will bereduced and the effectiveness increased substantially.

After the completion of the precarburization treatment, the billet,without quenching, and at substantially within the carburizationtemperature range of about 1600 and 2300 F. is rolled down or formed tothe required dimensions, and shapes, and then, if desired, subjected todifferential plastic deformation, as described fully in my Patent No.3,017,697.

Thus there is provided by the present invention, a method of producinghollow drill steel and other hollow steel members having a 'hard surfacecharacterized by high fatigue and corrosion resistance which comprisesthe steps of heating a hollow billet to a temperature between about 1600F. and about 2300 F. and above the transformation point of the steel ina carbon rich medium, maintaining the billet at that temperature in thecarbon rich medium for a time sufficient to effect carburization of atleast the exposed inside surfaces of the billet, and thereafter, withoutquenching, and at substantially the carburization temperature range,mechanically deforming the billet, to materially alter itscross-sectional area and to effect grain refinement and distribution ofcarbon and other alloying elements at the carburized surface.

There is also provided by the present invention a method of producinghollow drill steel and other hollow steel members having a hard surfacecharacterized by high fatigue and corrosion resistance which comprisesthe steps of heating a billet at a temperature at least about 100 F.above its transformation point with an alloying element other thancarbon, such as Cr, Mo, Si, Ni, V, Co, U, Ti, Cb, B or W, or theiralloys, to effect diffusion of the element into the billet surface, thenheating the billet to a temperature between about 1600" F. and 2300 F.,maintaining the billet at this temperature for a time sufficient toeffect diffusion into at least the exposed inside surfaces of thebillet, and thereafter, without quenching, and at substantially thediffusion temperature range, mechanically deforming the billet, tomaterially alter its cross-sectional area and to effect grain refinementand distribution of carbon and other alloying elements.

The novel concept of high temperature carburization or diffusionfollowed by hot finishing at temperatures close to those ofcarburization or diffusion, permits very rapid carburization ordiffusion as compared with prior art methods, drastically reducingcarburization time, for example, from usual 8 to 12 hours, to as littleas 30 minutes, because the very detrimental coarse grained structurebuilt up during intensive carburization at high temperatures issubsequently refined, in accordance with the invention, by the rollingor other mechanical deformation step at substantially the carburizationtemperature range. This procedure is illustrated in Example 6.

Precarburization results in a surface layer which may have a carboncontent up to about 1.3% C, thus provid ing sufficient carbon forfurther diffusion into the interior of the drill steel. Such diffusion,which may be achieved by further heat treatments so that the originalrelatively shallow high carbon case of 1.3% C and about 0.015 inch depthcan be extended to a deeper case of about 0.9%- 1.0% carbon and about0.040 inch depth, e.g., by heating at 1600 to 1900 F. for 2 to 6 hoursto achieve diffusional redistribution of the carbon. Suchprecarburization, and diffusion treatments may be multiplied as desired,and can be interspaced with other deformation and rolling treatments. Bycreating a carbon gradient, precarburization and prediffusion providereinforcement of the surface and allow deeper penetration of thelocked-in stresses to prevent premature failure of the steel, byreducing substantially the amount of retained soft austenite.

A fifth aspect of the invention is double or multiple carburization,i.e., precarburization followed by diffusional redistribution of thefirst case and a secondary carburization cycle resulting in double-plyor multiple carburization or preimpregnation with other than carbonmedia, which will provide additional reinforcement of the surface oftreated element.

Double-ply carburization involves, first a precarburization treatment;usually a high-temperature treatment applied in the upper limit ofpermissible temperature range 1750-2100 F., in some cases up to 2300 F.,in order to save on time and increase the intensity and carbongradients. This is followed by prediffusion treatment and afterwards 'bya substantial rolling and forming operation.

Manufactured in this way a bar, tube, or the like, after cutting andmachining, is exposed to a final carburizing treatment (secondcarburization), applied in the lower temperature range 1650 to 1750 F.,in order to limit the grain growth, during which treatment the secondcarbonply is built up.

Double-ply carburized surfaces are recommended for elements exposed tosevere working conditions such as ball and roller bearings, its races,drill steel, heavily stressed bolts, etc.

Double-ply or multi-ply precarburization and/or impregnation treatments,or straight double ca-rburizingdiffusion-impregnation-treatments, ifrequired, followed by spiral-rolling will result in the most durable andfatigue resistant surfaces.

The double-ply or multiple carburization and impregnation-diffusiontreatment described above amounts, in simple terms, to variouscombinations of proposed surface treatments such as, for example, one ortwo layers of carburized case combined, if required, with one or twolayers of impregnated and diffused cases of elements other than carbonand their alloys, interspaced as would be desired, e.g., carbon casefollowed by impregnated metal of about 0.015 to 0.030".

7 case and so on, applied before, during or after the final formingprocesses.

In the manufacture of solid drill steel, the precarburi- Zation and/ orpreimpregnation step is applied only to the outside surface of steelbillets used to manufacture hexagonal, quarter-octagon, and round crosssections of drill steel of diameter from A" to 2 /2", or to drill pipesof outside diameter from 2" to 6" and wall thickness from M1" to 1" andlength from 2 to 30 feet and is illustrated by the following example:

Example 2 A drill steel billet of a medium carbonnickelcbromium-molybdenum steel, 6" square and 40" long, was carburizedby heating in a bed of powdered coke for about 6 hours at a temperatureof about 1800 F., to form a hardened case of a depth between about 0.015and 0.060". The billet was removed, and rolled to form a round 1.5"diameter drill rod, and then allowed to cool.

There were then applied to the outer surface of the rod by means ofrolls, circumferential grooves or depressions of a hollow configuration,alternating with circumferential crest portions, the grooves having adepth of about & and the distance between grooves being about Theresulting drill rod exhibited improved physical and mechanicalproperties over standard drill rod stock.

In the new and improved process as applied to the man ufacture of hollowdrill steel, i.e., containing an inside longitudinal hole, theprecarburization and/or other preimpregnation and predilfusion steps areapplied to both the inside hole and the outer surface simultaneously orseparately. Such hollow drill steel may be prepared, for example, bystarting with a drill steel billet 4 to 10" square and 30" to 50" inlength, drilling a large hole (1" to 3" in diameter) through the centerwith a mechanical drill to allow for free access of the carburizingmedium. The billet is then precarburized and/or preimpregnated by anysuitable method, a suitable mandrel or other means of maintaining theinside geometry of the hole is inserted, and the billet is rolled to alength of 20 to 30 feet, and a diameter (round, hexagonal, orquarteroctagon) of 1- 1.5". Alternatively, if required, the billet withcentral mandrel therein may be rolled to the desired shape and lengthand then carburized after the mandrel is removed.

The process for hollow drill steel is illustrated by the followingexamples:

Example 3 A drill steel billet of low carbon nickel-chromiummolybdenumsteel having the analysis previously disclosed, 6 inches square and 40"long, and drilled to produce a central hole 2" in diameter, isprecarburized with a charcoal-barium carbonate mixture at a temperatureof 2000 F. for about 4 hours until the carburized case attained a depthbetween 0.040" and 0.080 on both the inside and outside surfaces. Aftercompletion of the precarburization, the billet is cooled and the insideand outside surfaces cleaned. An austenitic manganese steel rod 2" indiameter is inserted into the center hole of the billet in order tomaintain the inside geometry of the hole. The billet is then heated to arolling temperature of about 19 00 F. for about 2 hours, and rolled downto a 1" quarter-octagon section with inside hole of about diameter, andthen air colled to a surface hardness of 50-60 Rockwell C units. Themanganese steel rod is removed and the drill rod cut into the requiredlengths. In some instances the time of heating for rolling may beextended and the billet held for 4 to 6 hours at rolling temperatures todiffuse the carbon deeper into the interior ,of the billet and to widenthe case from 0.060" to 0.120".

After the completion of this additional prediffusion during the extendedheating cycle, and after the rolling operation, the precarburized,predilfused case will have a depth The drill rod lengths if required arethen subjected to surface differential plastic deformation as describedin Example 2.

The precarburization, predifiusion and other preim pregnation treatmentsof the invention are especially well suitable for treatment ofinaccessible surfaces, openings, channels, and the like, before rollingis applied to members containing these, for instance conical or threadeddrill rod attachments. Here the precarburized drill steel is heated atboth ends and subjected to forging and machining operations. Aftercompletion of these operations, in order to restore the carbon case atboth ends, a final gas carburization is applied at a lower temperaturefrom about 1600 to about 1700 F. for 6 to 8 hours, followed by oilquenching and drawing at 450 F. for 1 hour.

Differential plastic deformation, that is the impression of shall-owdepressions at spaced intervals on the surface of treated pieces, ifrequired, can be basically applied in three ways:

(1) Simple differential plastic deformation, by grooving the surface,applied at room temperature, i.e., by cold working, or else applied at atemperature below the recrystallization temperature of the metal, whichfor ferrous metals is usually below 1000 F.

(2) Differential plastic deformation at high temperatures lying abovethe recrystallization temperature and above the full stress relieftemperature of the metal. This is followed by overrolling with a rollhaving a smooth surface at room temperature or at a temperature belowrecrystallization and full stress relief temperature, viz, about 1000 F.

(3) Differential plastic deformation as described above at (1) combinedwith overrolling with a roll of smooth surface at room temperature or ata temperature below the recrystallization temperature and full stressrelief temperature of the metal.

The machines and rollers used in differential plastic deformation aredescribed in my copending application Serial No. 551,455, now Patent No.3,017,697, and this description is incorporated herein by reference. Thesmooth rolls are used for smoothing or overrolling the differentialplastic deformation patterns impressed during that stage of the process.

In accordance with a sixth aspect of the invention, therefore,precarburization and/or preimpregnation followed, if required, bydiffusion treatments, or standard carburization precedes differentialplastic deformation, and the latter step is followed by overrolling.This last sequence is illustrated by the following example:

Example 4 A A2" hexagonal 1080 SAE carbon drill steel rod in theas-rolled condition, without heat treatment, and having the composition:C 0.77, Mn 0.27, Si 0.19, P 0.018, S 0.025, was subjected todifferential plastic deformation as described in Example 1 by impressing24 grooves per inch on the drill steel having a groove depth between0.004" and 0.006" with a roll at a pressure of 550 pounds, with radiusat the rolling edge of the rolls. Overrolling was then applied withsmooth rolls with /4" radius at the rolling edge and 550 poundspressure. This resulted in a very substantial increase in fatiguestrength. Thus, under a rotating bending moment of 5000 inchpounds andmaximum surface stress about 61,500 pounds per sq. inch the standarduntreaded steel resisted 90,000

' cycles to failure, as compared with 1,055,000 cycles for 9vdeformation at high temperatures lying above the recrystallizationtemperature and the full stress relief temperature of the metal,generally above 1000 F., followed by over-rolling with smooth rolls at atemperature below the 1000 F. temperature. In this treatment the plasticdeformation grooves are applied while the metal is hot, at a temperatureabove the recrystallization and stress relief temperature. Stress relieftemperature may be defined as the temperature at which the effects ofcold working and the locked-in residual stresses resulting therefrom,

begin to disappear. The grooved surface is then overrolled at roomtemperature as described previously. This over-rolling eliminatesprotruding crests which are partially flattened, elaving the adjacentvalleys with lesser plastic deformation.

In accordance with still another aspect of the invention, there iscontemplated the combination of plastic deformation either at ordinarytemperature or at high temperatures (above 1000 F.), followed byover-rolling, the precarburization, preimpregnation or prediffusion stepbeing omitted. The benefits arising from the comb-ination ofover-rolling with either of these two types of plastic deformation havebeen pointed out. It will be understood, however, that the properties ofthe final product are nevertheless still further improved whenprecarburization and/ or preimpregnation is included in the process ofthe invention, for the reasons stated below.

The application of differential plastic deformation to the surface ofdrill steel, or hollow drill steel, increases its fatigue resistance andcorrosion-fatigue resistance so effectively that on the average, athreefold increase in drilling life is achieved. 9

Differential plastic deformation of precarburized and/ or prediffusedhollow drill steel is considerably more efiective in increasing fatigueresistance. This is because the presence of the carburized outersurface, supplemented by the strengthening effect of differentialplastic deformation, provides an enchancing of the fatigue resistancebeyond that of the additive effect of these two operations. Thesimultaneous precarburization of the inner hole portion of the hollowdrill rod acts to prevent the initiation of fatigue cracks and radiationthereof to the outer portions of the hollow rod. Thus, tests dataindicate in the case of a 4;" hexagonal low carbon Ni-Cr-Mo hollow drillsteel rod, which has been carburized on inside and outside surfaces,under a rotating bending moment of 7000 inch-pounds and maximum stressof :99,000 pounds per sq. inch, the rod which was not plasticallydeformed after carburization withstood 97,000 cycles, Whereas the samerod after carburization followed by plastic deformation, withstood950,000 cycles, a tenfold improvement. This clearly demonstrates theunexpected improvement conferred by the combination of precarburizationand plastic deformation, in accordance with the present invention. Thefatigue strength of the outside surface is supplemented by thereinforcing effect of the uniformly precarburized inside hole to providea superior drill steel. Similarly, precarburized conical drill rodattachments with plastic deformed (spiral rolled) conical tapers aremuch stronger than when not precarburized.

Example 5 This example was performed for the illustration of theprecarburization, prediffusion and double-ply carburization process onthree one inch square and hollow samples of low carbon (0.20-0.30% C)nickel-chromium-molybdenum steel having the analysis as specified above.

All three samples were precarburized in a gas carburizing furnaceemploying hydro carbon mixtures at a temperature of 1725 F. for sixhours then air cooled and reheated for the standardization of thestructure to 1550 F. for 45 minutes and air cooled, the precarburizedcase was about inch thick and show surface hardness of 55-60 Rockwell C.

After completion of the precarburization the first sample was thenheated to a rolling temperature of about 1900 F. for about one hour androlled down to inch and then air cooled to a surface hardness of about55 Rockwell C units. The thickness of the precarburized case afterrolling operation was then about 10 to inch and, as was to be expectedaccording to the invention, evenly distributed on the inside and outsidesurfaces and microstructure refined according to the invention. Thesecond sample was precarburized in the same way as the first sample andadditionally prediffused at a temperature of 1700" F. for 4 hours inprotective atmosphere and air cooled to surface hardness of about 55Rockwell C units. The precarburized case was then prediffused fromoriginal thickness of inch to inch, i.e., it increased by about 50%showing a smoother and better redistribution of carbon and a substantialdecrease of soft retain austenite on the inside and outside surface ofthe sample. After completion of the precarburization and prediffusiontreatments, the second sample was heated to a rolling temperature ofabout 1900 F. for about one hour and rolled down to 7 inch and then aircooled to a surface hardness of 50-55 Rockwell C units. The thickness ofthe precarburized and prediffused case after rolling was about A inchand the inside and outside case showed all improvements provided by theinvention. The third sample was precarburized and prediffused in asimilar way as the second sample, but before final rolling operation wasprecarburized for the second time at 1725 F. for two hours and aircooled in order to demonstrate the so called double-ply carburization,i.e., the reinforcement of the treated surface with two precarburizedlayers interspaced by a preditfusion treatment. Before the final rollingoperation the second precarburized layer was inch thick and the firstprecarburized layer was effectively diffused into the surface by aboutinch thick. After final rolling the first carburized case was about inchthick and second case about 7 inch. These three tests proved, in allaspects listed above, the effectiveness of the precarburization,prediffusion and double-ply carburizati'on in the manufacture of durabledrill steel. In all three tests according to the invention, it wasproved that a very effective grain refining and better redistribution ofcarbon and other alloying elements takes place resulting in a verysubstantial fatigue and corrosion fatigue strength of structural membersmanufactured according to this invention.

Example 6 A drill steel billet of low carbon nickel-chromiummolybdenumsteel having the analyses previously disclosed. 8 inches square 40"long, and drilled to produce a central hole 3 inches in diameter, isprecarburized at a temperature of 2300 F. for about 30 minutes until thecarburized case attains a depth between 0.040" and 0.080" on both theinside and outside surfaces.

After completion of the precarburization the inside and outside surfaceis cleaned and an austenitic manganese steel rod 3 in diameter isinserted into the center hole of the billet in order to maintain theinside geometry of the hole and afterwards without quenching and atsubstantially the hot rolling temperature range 1900 to 2300 F. isrolled down to 1 hexagonal section and cooled to a surface hardness of45 to 55 Rockwell C units.

I claim:

1. Method for improving the fatigue resistance of a drill steel memberwhich comprises the steps of carburizing the exposed surfaces of saidmember by heating in a medium rich in carbon at a temperature between1600 and 2300 F., for a period of time sufiicient to effectcarburization of said exposed surfaces, then subjecting the outercarburized surface to plastic deformation by cold working by applying toportions of said surface and at spaced intervals, pressure beyond theelastic limit of the steel, said pressure being sufficient to providesmall plastically deformed depressions at intervals, said intervalsbeing interspaced with substantially undeformed portions, the distancebetween said depressions being greater than the depth of saiddepressions, and further subjecting said undeformed portions toover-rolling with a smooth roll at a temperature between roomtemperature and about 1000 F.

2. Method for improving the fatigue resistance of a drill steel memberwhich comprises the steps of carburizing the exposed surfaces of saidmember by heating in a medium rich in carbon at a temperature betweenabout 1600" and 2300" F., and above the transformation point of thesteel for a period of time sufficient to effect carburization of saidexposed surfaces, then subjecting the outer carburized surface toplastic deformation at a temperature above 1000 F. by applying toportions of said surface and at spaced intervals, pressure beyond theelastic limit of the steel, said pressure being sufficient to providesmall plastically deformed depressions at intervals, said intervalsbeing interspaced with substantially undeformed portions, the distancebetween said depressions being greater than the depth of saiddepressions and then subjecting the surface to over-rolling with asmooth roll at a temperature between room temperature and about 1000 F.

3. Method for improving the fatigue resistance of a drill steel memberwhich comprises the steps of subjecting the outer surface of said memberto plastic deformation by cold working by applying to portions of saidsurface and at spaced intervals, pressure beyond the elastic limit ofthe steel, said pressure being sufficient to provide small, plasticallydeformed depressions at intervals, said intervals being interspaced withsubstantially undeforrned portions, the distance between saiddepressionsbeing greater than the depth of said depressions, and then subjectingsaid member to over-rolling with a smooth roll.

4. Method for improving the fatigue resistance of a drill steelmemberwhich comprises the steps of subjecting the outer surface of said memberto plastic deformation at a temperature above about 1000 F. by applyingto portions of said surface and at spaced intervals, pressure beyond theelastic limit of the steel, said pressure being suflicient to providesmall, plastically deformed depressions at intervals, said intervalsbeing interspaced with substantially undeformed portions, the distancebetween said depressions being greater than the depth of saiddepressions, and then subjecting said member at a temperature betweenroom temperature and about 1000 F. to over-rolling with a smooth roll.

5. Method for producing hollow steel members comprising heating a shortbillet of large cross-sectional area and having a large bore to atemperature above the transformation point of the steel in a carbon richmedium and maintaining the billet at that temperature for a period oftime sufficient to effect substantially uniform carburization anddiffusion of hardened elements into both the inner and outer surfaces ofsaid billet, then hot-rolling said billet to reduce the cross-sectionalarea and increase the length of the billet several fold and to effectgrain refinement and longitudinal orientation, and then subjecting theouter surface 'of the rolled member at spaced intervals to theimpression of shallow plastically deformed depressions about onesixty-fourth of an inch deep, said intervals being spaced withsubstantially undeformed portions, the dis tance between saiddepressions being about one twentyfourth of an inch.

6. The method of claim 5 in which the member, after the impression ofshallow plastically deformed depressions, is further over-rolled at atemperature below the recrystallization temperature of the steel.

7. The method of producing hollow drill steel and other hollow steelmembers having a hard surface characterized by high fatigue andcorrosion resistance which comprises the steps of heating a hollowbillet to a temperature between about 1600 and 2300 F. and above thetransformation point of the steel in a carbon rich medium, maintainingsaid billet at said temperature in the carbon rich medium for a timesufficient to effect carburization of at least the exposed insidesurfaces of the billet, and thereafter, without quenching, and atsubstantially the said carburization temperature range, mechanicallydeforming the billet, to materially alter the cross-sectional areathereof and effect grain refinement and distribution of garbon and otheralloying elements at the carburized surace.

8. The method of producing hollow drill steel and other hollow steelmembers having a hard surface characterized by high fatigue andcorrosion resistance which comprises the steps of heating a billet at atemperature at least about F. above its transformation point with analloying element other than carbon to effect diffusion of said elementinto the billet surface, then heating the billet to a temperaturebetween about 1600" to 2300 F., maintaining said billet at saidtemperature for a time sufficient to effect diffusion of at least theexposed inside surfaces of the billet, and thereafter, withoutquenching, and at substantially the said diffusion temperature range,mechanically deforming the billet to materially alter the crosssectionalarea thereof and effect grain refinement and distribution of carbon andother alloying elements.

References Cited by the Examiner UNITED STATES PATENTS 4/1959 Thorn eta1 l48-l2.1 1/ 1962 Wlodek 72377

1. METHOD OF IMPROVING THE FATIGUE RESISTANCE OF A DRILL STEEL MEMBERWHICH COMPRISES THE STEPS OF CARBURIZING THE EXPOSED SURFACES OF SAIDMEMBER BY HEATING IN A MEDIUM RICH IN CARBON AT A TEMPERATURE BETWEEN1600* AND 2300*F., FOR A PERIOD OF TIME SUFFICIENT TO EFFECTCARBURIZATION OF SAID EXPOSED SURFACES, THEN SUBJECTING THE OUTERCARBURIZED SURFACE TO PLASTIC DEFORMATION BY COLD WORKING BY APPLYING TOPORTIONS OF SAID SURFACE AND AT SPACED INTERVALS, PRESSURE BEYOND THEELASTIC LIMIT OF THE STEEL, SAID PRESSURE BEING SUFFICIENT TO PROVIDESMALL PLASTICALLY DEFORMED DEPRESSIONS AT INTERVALS, SAID INTERVALSBEING INTERSPACED WIH SUBSTANTIALLY UNDEFORMED PORTIONS, THE DISTANCEBETWEEN SAID DEPRESSIONS BEING GREATER THAN THE DEPTH OF SAIDDERPRESSIONS, AND FURTHER SUBJECTING SAID UNDEFORMED PORTIONS TOOVER-ROLLING WITH A SMOOTH ROLL AT A TEMPERATURE BETWEEN ROOMTEMPERATURE AND ABOUT 1000*F.